Requests from users for electrophotography to be employed in, for example, a receiving apparatus of a copying machine, printer, or facsimile machine have become more demanding year by year in association with the development of the apparatus. The recent trend is as follows: the electrophotography has been strongly requested to be capable of performing printing on a large number of sheets and providing stable image quality independent of an environment in association with the expansion of the variety of environments where the electrophotography is employed owing to the expansion of markets.
In order that the above requests may be satisfied, a need for toner having high durability and capable of providing high image quality has become even higher than before, and a large number of improvements have been made with a view to solving the foregoing problems.
For example, it has been known that a fatty acid metal salt is incorporated into toner so as to be used as a cleaning assistant or an agent for preventing filming on an electrostatic latent image-bearing member. On the other hand, however, an improvement in image quality cannot be achieved because the fatty acid metal salt causes fogging or a reduction in image density. In view of the foregoing, the following has been disclosed (for example, Japanese Patent Application Laid-Open No. H08-272132): the fatty acid metal salt and a titanate compound are used in combination so that fogging may be alleviated while filming on the electrostatic latent image-bearing member and voids are alleviated.
In addition, the following has been disclosed (for example, Japanese Patent Application Laid-Open No. H09-311499 and Japanese Patent Application Laid-Open No. 2002-296829): a relationship among the particle diameter of toner particles having a certain storage modulus or the grain size distribution of the toner particles, and the particle diameters and grain size distribution of the particles of a fatty acid metal salt is specified so that image quality may be improved, and fogging and filming on an electrostatic latent image-bearing member may be alleviated.
In addition, the following has been disclosed (for example, Japanese Patent Application Laid-Open No. 2007-148198): fogging, toner scattering, and toner leakage are suppressed by incorporating additives (alumina and titanium oxide) having a specified relationship of a work function to base particles, and a fatty acid metal salt.
Further, the following has been disclosed (for example, Japanese Patent Application Laid-Open No. 2007-108622): a toner particle is coated with a fatty acid metal salt, suppressing the liberation ratio of the fatty acid metal salt so that image stability may be improved while the fatty acid metal salt can serve as an agent for inhibiting filming on an electrostatic latent image-bearing member.
Indeed, such measures have allowed fogging, toner scattering and toner leakage to be reduced while suppressing the filming of toner on an electrostatic latent image-bearing member, and high durability and high image quality stability to be obtained. However, as a result of the extensive studies conducted by the inventors of the present invention, it has been found that the particle diameter of the particles of the fatty acid metal salt to be used in the toner described in each of Japanese Patent Application Laid-Open No. H08-272132 and Japanese Patent Application Laid-Open No. H09-311499 is so large that a problem is raised in that although the fatty acid metal salt has a certain effect on initial fogging, a change in chargeability of the toner becomes large and fogging occurs when printing is performed on a large number of sheets. Additionally, it has been found that the toner described in each of Japanese Patent Application Laid-Open No. 2002-296829 and Japanese Patent Application Laid-Open No. 2007-148198 involves the following problem: under a severe environment such as a low-temperature, low-humidity environment or a high-temperature, high-humidity environment, an increase in the number of sheets on which printing is performed leads to the occurrence of fogging due to deterioration in the chargeability of the toner.
In addition, it has been found that the toner described in Japanese Patent Application Laid-Open No. 2007-108622 involves the following problem: toner particles must be coated with the fatty acid metal salt, and hence, large mechanical damage is done to toner particles in the coating step, and a development stripe is apt to occur.
Moreover, it has been found that each of the measures still involves a problem in terms of the contamination of a member in which rubbing largely occurs, such as a toner bearing member or a toner-supplying member, with toner under a low-temperature, low-humidity environment in addition to the above-mentioned problems involved in the measures. It has also been found that, particularly when printing is performed on a large number of sheets, detrimental effects on an image resulting from the filming of the toner on the member occur. At present, improvements in various characteristics have still been needed in order that stable developability independent of environments in which the toner is used can be obtained even when printing is performed on a large number of sheets, as requested in the market.